We shall employ our recently developed fluorescent marker for Merkel cells, the dye quinacrine, as a light microscopic means of identifying these mechanosensory target cells to aid the subsequent sampling of tissues for electromicroscopic examination following their physiological and pharmacological manipulation. The latter include a variety of approaches that may deplete Merkel cells (in skin of salamanders and of Xenopus) of their granules, and/or the nerve endings of their vesicles; such depletion is expected to bring about alterations in the mechanosensory function of the neurite-Merkel cell complexes. The manipulations include repeated mechanical and electrical stimulation, changes in ionic concentrations, application of drugs, venoms, and toxins, nerve section or treatment with colchicine. Some of these treatments will be applied in vitro on "isolated nerve-skin" preparations, and on co-cultures of Xenopus epidermis and sensory ganglia (dissociated cells and explants); the latter in vitro studies will include the investigation of the target role of Merkel cells and their involvement in neurite outgrowth, and examination of "conditioned medium" for possible diffusible agents responsible for these trophic phenomena.